1. Field of the Invention
The present invention relates in general to a clutch structure installed with an automatic transmission, and more particularly to a clutch structure employed in automotive automatic transmissions of a type which has two clutches coaxially overlapped.
2. Description of the Prior Art
In an automatic transmission, there are installed various frictionally engaging elements, such as clutches, brakes and the like, which constitute part of a gear train of the transmission. Under operation of the transmission, for achieving gear changes, such elements are controlled in ON-OFF (viz., engaged-disengaged) manner by fluid pressures produced through control valves in accordance with the running condition of a motor vehicle.
Among the conventional automatic transmissions hitherto proposed and put into practical use, there is a type in which two clutches are coaxially overlapped for the purpose of reducing the size, viz., the axial length, of the transmission. Japanese Patent First Provisional Publication No. 55-40367 shows one of the transmissions of such type, in which a clutch drum of a high clutch is coaxially received in a clutch drum of a reverse clutch.
In the transmission of the publication, a piston of the reverse clutch is coaxially received in the clutch drum of the same in a manner to face the under surface of the clutch drum of the high clutch. Drive and driven clutch plates are juxtaposed within a cylindrical clearance defined between the inner and outer surfaces of the respective clutch drums of the reverse and high clutches, and, upon application of fluid pressure to a piston working fluid chamber, these plates are compressed by the piston thereby to achieve engagement between these two clutch drums. A plurality of return springs are arranged to lie about a cylindrical inner wall portion of the piston of the reverse clutch in a space defined between the piston and the under surface of the clutch drum of the high clutch. One end of each return spring is pressed on the piston. An annular spring retainer for retaining the other end of each return spring is securedly mounted to a fixed shaft portion along which the piston moves.
However, because of provision of the return springs and the spring retainer at such a position as described hereinabove, the axial length of the space is remained considerable even when the piston comes closest to the under surface of the clutch drum of the high clutch. That is, it is inevitably necessary to increase the axial length of the space to a degree greater than the stroke of the piston. This necessity causes the combined structure of the two clutch drums to have a considerable axial length irrespective of employment of the overlapped clutch arrangement.